Notes. At t ≲1 day, the blackbody peaks in the UV, while for every subsequent spectrum, both the Rayleigh-Jeans and Wien tails are constrained. In these subsequent spectra, dust extinction uncertainties contribute the dominant statistical uncertainty to the blackbody temperature. In Fig. 3, we show the best-fit blackbodies alongside the observed spectra.

(1) Shappee et al. (2017) – these values of T_obs remain unchanged as no spectral features have been noted in these epochs; (2) Andreoni et al. (2017) – as the ANU spectrum does not strongly constrain the continuum shape, we report T_obs for this epoch as constrained by preceding and subsequent photometry – which suggests cooling from 7600 to 6600 K between 0.7 and 1.0 days post-merger (see Drout et al. 2017; (3) Buckley et al. (2018); (4) Pian et al. (2017); (5) Sneppen et al. (2023a); (6) Nicholl et al. (2017) – which is contemporaneous with NIR spectra from Gemini/FLAMINGOS-2 (see Chornock et al. 2017); (7) Smartt et al. (2017).